1. Field of the Invention
The present invention relates to an image processing method and an ink jet printing apparatus, more particularly, it relates to an image processing method and ink jet printing apparatus for performing image processing for reducing a joining stripe caused by scans of a printing head in image printing.
2. Description of the Related Art
Recently, the need for high speed printing has increased in printing to a print medium by a printing apparatus. In an ink jet printing apparatus for printing by ejecting ink droplets, in order to print one line of dots constituting an image, a multi-pass printing method is employed for printing by making a printing head scan to a line a plurality of times.
The multi-pass printing method has an advantage that high image quality printing can be realized. However, in order to realize high speed printing, it is effective to reduce the number of passes that is the number of scans of a carriage necessary for completion of printing for one line. That is, since a paper conveyance amount per one time becomes small as the number of passes becomes large, the printing speed per one line is decreased. On the other hand, since the paper conveyance amount per one time becomes large as the number of passes becomes small, the printing speed per one line is increased. For example, comparing four-pass printing with two-pass printing, about two times the speed is realized if two-pass printing instead of four-pass printing is performed. Accordingly, as the number of passes becomes small, the number of scans of the carriage required for printing to a predetermined region of a piece of print paper or the like is decreased, and the paper conveyance amount per one time becomes large, and consequently a time required for a predetermined printing is shortened.
In the multi-pass printing method, since printing is performed to a region for one band, which is a band-shaped image region, by multiple scanning, a joining stripe is likely to be caused at a joining part between the bands.
FIG. 1 is a view showing an image region in which the two-pass printing is performed. In the ink jet printing apparatus, the carriage scans in a direction orthogonal to an arrangement direction of ink ejecting ports so that printing is performed, and thus a band-shaped image print region is formed every scanning. When printing is performed on the print medium, for example, plain paper on which ink easily bleeds, a black joining stripe caused by the bleeding of the ink at the joining part between the bands can be visually checked although the degrees of the stripes are different from each other depending on the properties of ink droplets or print mediums. In particular, when the number of passes is reduced and printing is performed, a clear joining stripe can be visually checked, and print quality is lowered. That is, the number of dots to be printed on the print medium by the scanning in two-pass printing becomes about twice that in four-pass printing, and consequently duty, which is the rate of dots to the region, also becomes approximately twice. Therefore, a clearer black joining stripe can be visually checked as the number of passes becomes smaller.
As a method for realizing a high image quality by removing the black joining stripe, a method disclosed in Japanese Patent Laid-Open No. 2002-036524 is conventionally known that printing is performed per a band by repeating main scans of the printing head, and that print data is subjected to a thinning-out processing corresponding to a print amount of a region in the vicinity of the joining part. More specifically, dots printed on the region in the vicinity of the joining part are counted, and the thinning-out processing is performed corresponding to the counted value.
Additionally, in Japanese Patent Laid-Open No. 8-025693 (1996), it is disclosed that the joining stripe is reduced in a one-pass printing method for printing to a certain region by making the carriage scan only once. That is, a printing method is disclosed that images to be each printed by scanning are partly overlapped with each other, and complement the overlapped region by a random mask pattern between scanning.
However, the conventional method disclosed in Japanese Patent Laid-Open No. 2002-036524, is applicable to the case where the joining stripe is black, but not applicable to the case where the joining stripe is white. The joining stripe is caused by low precision of the paper conveyance, end nozzle dot deflection or the like unique to the printing apparatus in addition to the ink bleed, and the white joining stripe is sometimes caused at the joining part. However, the conventional method cannot solve the above problem. Here, the end nozzle dot deflection indicates a phenomenon that ink droplets landing position by a nozzle arranged on both ends of a plurality of arranged nozzles deviate toward a center axis of the printing head. Regarding the end nozzle dot deflection phenomenon, it is observed that the deflection of the landing position of the ink droplets is large as the duty in each scanning is high, or the size of the ink droplets is small. In particular, when the ink droplets are not more than 2.8 pl, the end nozzle dot deflection becomes large. Accordingly, the end nozzle dot deflection becomes large in the two-pass printing since duty of ink in the two-pass printing is larger than that in the four-pass printing. Thus, the white joining stripe is likely to occur at the joining part between the bands. In particular, a clear white joining stripe can be visually checked on a print medium such as photo-paper on which the ink hardly bleeds, and the image quality is lowered.
Additionally, in the conventional methods disclosed in Japanese Patent Laid-Open Nos. 2002-036524 and 8-025693 (1996), image data for the nozzles is corrected, the nozzles being arranged at both ends of the nozzle arrangement line. In this case, since the ink droplets ejected from the nozzles arranged at both ends are affected by the end nozzle dot deflection, a desired correction is hardly applied to the image data, and the joining stripe is not always reduced. In particular, such precision is insufficient for printed images such as photographs and graphics for which a high quality image is required.